The present invention relates to a method and apparatus for reconstructing images from total internal reflection holograms and particularly for increasing the effective reconstruction efficiency of total internal reflection holograms.
The principles of total internal reflection (TIR) holography are described in U.S. Pat. No. 4,857,425 since which time many advances have been made to maximise the advantages of TIR holography when applied to microelectronics manufacturing. Other prior art references are U.S. Pats. Nos. 4,917,497, 4,966,428, 5,187,372, 5,640,257, 5,695,894 and European Patent Appl. No. 98303677.
U.S. Pat. No. 4,966,428 discloses an apparatus based on TIR holography for manufacturing integrated circuits wherein a holographic image formed in a first recording medium on a glass plate is replayed by scanning it with a narrow collimated beam of light so as to print the image into a second recording medium on a silicon wafer arranged in proximity and parallel to the first recording medium. A xe2x80x9czigzagxe2x80x9d or raster scanning scheme is disclosed wherein the beam scans in lines of alternate direction with a partial overlap between successive scan lines whereby the complete holographic image recorded in the first recording medium is printed into the second recording medium. This scanning technique provides a uniform time-integrated illumination of the hologram and also maximises the power of the replay beam arriving at the hologram in order to minimise the replay time.
U.S. Pat. No. 5,695,894 discloses a TIR holographic system for changing the scale of a pattern reconstructed from a TIR hologram wherein a first computer controlled stage produces a raster scan of the illumination beam across the hologram while a second computer controlled stage simultaneously displaces a photoresist coated wafer relative to the hologram in orthogonal directions in the plane of the wafer. Changing the scale of the pattern printed from the hologram onto a wafer is important for a multi-level process in which a pattern has to be printed onto a wafer with a pattern already printed on it such that the two patterns are accurately registered, or aligned, with respect to each other. In such cases there may exist small differences in the corresponding dimensions of the respective patterns arising from, for example, an irreversible expansion of the wafer caused by a high-temperature, post-exposure treatment. To counteract this so that accurate overlay is achieved over the complete pattern, the difference in scale between the patterns on the wafer and in the hologram is first determined by measuring the relative positions of reference alignment marks included alongside the respective patterns and then the difference is compensated by providing a relative motion between the wafer and hologram as the illumination beam scans during the printing. Since this procedure produces some loss in resolution of printed image, U.S. Pat. No. 5,695,894 additionally teaches that an amount of convergence or divergence be introduced into the illumination beam so that the image instantaneously reconstructed from the hologram by the scanning beam is itself magnified or demagnified by the measured amount. For this purpose an apparatus is disclosed comprising a beam decollimator and a prism through which the beam passes before it illuminates the hologram. Adjustment of the separation of lenses in the beam decollimator changes the degree of convergence or divergence of the beam whereas the prism compresses the beam in one plane so that the magnification or demagnification of the image reconstructed from the hologram is rendered isotropic.
EP-A-0421645 discloses the combination of a raster scanning of the illumination beam in a TIR holographic system with a continuous measurement of the local separation of the hologram and wafer surfaces at the location of the beam as it scans and a continuous adjustment of that separation to the focal distance of the image reconstructed from the hologram in order that the image is accurately printed in focus on the wafer surface. This technique overcomes the problem caused by the unevenness of wafer surface and the limited depth-of-focus of high-resolution images.
European Patent Application No. 98303677 discloses an enhancement of the previous method to further improve the accuracy with which the image is printed in focus on the wafer surface wherein the dimension of the collimated illumination beam in the scanning direction is additionally compressed relative to its dimension in the stepping direction.
A further important consideration for the application of TIR holography to microelectronic manufacturing is the time it takes to print a pattern from a hologram onto a wafer or other substrate. Using the raster scan techniques described in the prior art, the time, t, it takes to print a pattern of length, l, and width, w, from a hologram onto a substrate can be estimated as:                     t        =                              l            ⁢                          xe2x80x83                        ⁢            w                                v            ⁢                          xe2x80x83                        ⁢            s                                              equ        .                  xe2x80x83                ⁢                  (          1          )                    
where v is the scanning speed of the beam and s is the stepping distance between successive scan lines (for this estimation the time it takes for the beam to decelerate, step and then accelerate between scan lines has been neglected and the beam dimensions have been assumed to be negligible compared to the pattern size).
However, the scan speed and step size of the beam during the scanning also have to satisfy the condition:                     E        =                              η            ⁢                          xe2x80x83                        ⁢            P                                v            ⁢                          xe2x80x83                        ⁢            s                                              equ        .                  xe2x80x83                ⁢                  (          2          )                    
where E is the exposure dose required by the photoresist, P is the power of the laser beam at the output of the laser and xcex7 is the efficiency, or transmission, of the complete optical system between the laser and the wafer surface.
From equs. (1) and (2) it therefore follows that:                     t        =                              l            ⁢                          xe2x80x83                        ⁢            w            ⁢                          xe2x80x83                        ⁢            E                                η            ⁢                          xe2x80x83                        ⁢            P                                              equ        .                  xe2x80x83                ⁢                  (          3          )                    
Evaluating this for a pattern of dimensions 30 cmxc3x9740 cm, a photoresist of sensitivity 100 mJ/cm2, an argon laser of output power 2 W and an optical system efficiency of 50% produces:   t  =                    30        xc3x97        40        xc3x97        0.1                    0.5        xc3x97        2              =                  120        ⁢                  xe2x80x83                ⁢        s            =              2        ⁢                  xe2x80x83                ⁢        m        ⁢                  xe2x80x83                ⁢        i        ⁢                  xe2x80x83                ⁢        n        ⁢                  xe2x80x83                ⁢        u        ⁢                  xe2x80x83                ⁢        t        ⁢                  xe2x80x83                ⁢        e        ⁢                  xe2x80x83                ⁢        s            
Although this is acceptable for industrial application it would be advantageous if it could be reduced.
It is clear from equ. (3) that the time it takes to print a pattern of certain dimensions may be reduced by any of i) increasing the sensitivity of the resist, ii) increasing the transmission of the optical system, and iii) increasing the laser power. However, i) is usually difficult because photoresists are generally mature and optimised products, ii) is difficult because of the properties of TIR holograms, and iii) although feasible, in that an additional or more powerful laser source can be added to the system, may be unattractive because of operating costs.
It is therefore an object of the present invention to provide a method and apparatus to reduce the time required to print an image from a total internal reflection hologram, in particular it is an object to increase the effective reconstruction efficiency of a total internal reflection hologram.
According to the invention there is provided a method for reconstructing an image from a total internal reflection hologram that includes the steps of
arranging the hologram in relation to a first face of a coupling body;
generating a substantially collimated illumination beam;
directing the illumination beam through a second face of the coupling body so that it reconstructs the image recorded in the hologram;
recycling at least once the light in the illumination beam that does not reconstruct the imagine wherein the light reflected from the hologram is directed out of the coupling body through the second or a third face and is subsequently redirected as a recycled beam through the second face of the coupling body such that it also reconstructs the image recorded in the hologram;
scanning the illumination and recycled beams across the hologram.
The hologram is preferably arranged on a first surface of a transparent substrate such as a glass plate the other surface of which is brought into optical contact with the first face of the coupling body by way of a refractive index matching fluid. The coupling body preferably comprises a prism of a transparent material such as glass. A second and a third face of the prism are inclined relative to the first face in order that an illumination beam can be directed through the second face so that it reconstructs the image recorded in the hologram either directly or indirectly following reflection from another face or other faces of the prism and in order that the light reflected from the hologram leaves the prism through the second or the third face, either directly or indirectly. The coupling body may alternatively comprise a transparent substrate with a diffractive element of the type described in EP 98300188 on its second face whereby both the illumination beam and the beam reflected the hologram are coupled from and to the ambient by grating structures in the diffractive element. The hologram may alternatively be arranged directly on the first face of the coupling body by recording it in a photosensitive layer applied directly to that face.
Preferably the scanning comprises a sequence of alternating scan lines and scan steps as in a raster scan though other scanning schemes may be employed. In order that the images reconstructed by the illumination and recycled beams can be printed accurately and uniformly in focus onto a substrate using a technique such as that described in EP-A-0421645 it is desirable that the separation between the illumination and recycled beams at the hologram be minimised and that the continuous measurement of the separation of the hologram and substrate be determined at the midpoint of the beams. To further optimise the accuracy and uniformity of focus with which the images are printed onto the substrate, it is further desirable that the beam generation provides that the dimension of the illumination beam in the scanning direction be compressed relative to its dimension in the stepping direction, and that the recycling provides that the recycled beam at the hologram be spatially offset from the illumination beam in the scanning direction.
As the illumination beam scans across or along an edge of a pattern recorded in the hologram, the uniformity of intensity of the recycled beam will necessarily be degraded because the intensity of that part of the recycled beam reflected from outside of the pattern will be higher than that part of the beam reflected from inside of the pattern. This will give rise to a non-uniformity in the pattern printed from the hologram by the recycled beam which, depending on its magnitude, may be a problem. In order that the recycled beam provides a more uniform reconstruction of the image from the hologram the method of the invention may be enhanced by additionally inverting the recycled beam in the direction in which the illumination and recycled beams are offset. Thus, if the recycled beam is offset from the illumination beam in the direction of the scan lines, then the inversion should provide that the front of the recycled beam be derived from the back of the illumination beam, and vice versa. In this way the otherwise abrupt change in the time-integrated exposure energy density of the recycled beam reconstructing the hologram produced by the illumination beam traversing along or across an edge of the pattern recorded in the hologram is made gradual, and this allows it to be substantially or wholly eliminated by including the additional step of adjusting either the power, scanning speed or step size of the illumination beam as the illumination and recycled beams traverse the edge of the pattern.
In another embodiment the method of the invention includes spatially filtering the recycled beam before it reconstructs the image in the hologram so as to reduce or eliminate any optical noise introduced into beam by the reconstruction of the image from the hologram.
According to the invention there is also provided an apparatus for reconstructing an image from a total internal reflection hologram that includes:
a coupling body in relation to whose first face the hologram is arranged;
at least one light source for generating a light beam;
beam shaping optics for increasing the dimension of the light beam in at least one plane and for providing a substantially collimated illumination beam;
means for directing the illumination beam through a second face of the coupling body so that it reconstructs the image recorded in the hologram;
means for recycling at least once the light in the illumination beam that does not reconstruct the image wherein the light reflected from the hologram is directed out of the coupling body through the second or a third face and is subsequently redirected as a recycled beam through the second face of the coupling body so that it also reconstructs the image recorded in the hologram;
a means for scanning the illumination and recycled beams across the hologram.
Preferably the means for scanning the illumination and recycled beams is a single two-axis stage system that generates a two-dimensional scan pattern such as a raster scan composed of an alternating sequence of scan lines and orthogonal scan steps. Preferably the means for recycling the light provides that the illumination and recycled beams are spatially separated at the hologram and that their separation is minimised such that the images reconstructed therefrom can be printed accurately and uniformly in focus onto a substrate using, for example, the method described in EP-A-0421645. Advantageously the recycling means includes at least one optical element mounted to the scanning means that deflects the beam recycled from the coupling body so that it is directed parallel to the original illumination beam back through the second face of the coupling body to the hologram. The beam deflecting element may simply comprise a mirror that reflects the recycled beam such that is parallel and spatially offset from the original illumination beam, or it may comprise a pair of mirrors that additionally inverts the recycled beam in the direction in which the illumination and recycled beam are spatially offset. With such an inversion it is desirable that the apparatus additionally includes a means for adjusting the intensity, scanning speed or step size of the illumination beam for the purpose of enabling a uniform reconstruction of the image from the hologram at the edges of the hologram. The beam deflecting optical elements mounted to the scanning means may alternatively comprise a polarisation rotator and a polarising beamsplitter which combine the recycled beam with the original illumination beam so that the two are spatially superposed and collinear at the hologram.